A goal of many modem long haul optical transport systems is to provide for the efficient transmission of large volumes of voice traffic and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time division multiplexing (TDM) and wavelength division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalently wavelength, are co-propagate in the same fiber to achieve high spectral efficiency, high data rate transport.
The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two level binary modulation scheme is predominantly employed.
Typical long haul optical transport dense wavelength division multiplexed (DWDM) systems transmit 40 to 80 channels of 10 Gbps (gigabit per second) across distances of 3000 to 6000 km in a single 30 nm spectral band. A duplex optical transport system is one in which traffic is both transmitted and received between parties at opposite end of the link. In current DWDM long haul transport systems transmitters different channels operating at distinct carrier frequencies are multiplexed using a multiplexer. Such multiplexers may be implemented using array waveguide grating (AWG) technology or thin film technology, or a variety of other technologies. After multiplexing, the optical signals are coupled into the transport fiber for transmission to the receiving end of the link.
At the receiving end of the link, the optical channels are de-multiplexed using a de-multiplexer. Such de-multiplexers may be implemented using AWG technology or thin film technology, or a variety of other technologies. Each channel is then optically coupled to separate optical receivers. The optical receiver is typically comprised of a semiconductor photodetector and accompanying electronics.
The total link distance may in today's optical transport systems be two different cities separated by continental distances, from 1000 km to 6000 km, for example. To successfully bridge these distances with sufficient optical signal power relative to noise, the total fiber distance is separated into fiber spans, and the optical signal is periodically amplified using an in line optical amplifier after each fiber span. Typical fiber span distances between optical amplifiers are 50-100 km. Thus, for example, 30 100 km spans would be used to transmit optical signals between points 3000 km apart. Examples of inline optical amplifiers include erbium doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs).
Alternatively, a Raman optical amplifier may be used to boost the optical signal power. Most Raman optical amplifiers comprise at least one high power pump laser that is launched into the fiber span. Through the nonlinear optical process of stimulated Raman scattering in the SiO2 of the glass of the fiber span, this pump signal provides gain to the optical signal power. A Raman amplifier may be co-propagating or counter-propagating to the optical signal, and a common configuration is to counter-propagate the Raman pump. A Raman amplifier may be used alone, or in combination with an alternate example of an inline optical amplifier, such as an EDFA. For example, a Raman amplifier may be used in conjunction with an inline optical amplifier to accommodate high loss spans and to bring the net span loss within an allowable system dynamic range.
When a plurality of high power Raman pump lasers are used it is desirable to control the gain. An example of Raman amplifier gain control in the current art is to measure the input signal power and output signal power and to compare these measurements. In a Raman amplifier that uses the fiber span, this control method is slow because of the large distance between the input and the output of the fiber span. Consequently there is a need for a fast Raman gain control method, and in particular a Raman gain control method that is based on power measurements on only one end of the transmission fiber span.
In another Raman amplifier gain control method currently practiced, the gain was controlled with a probe wavelength. This method is not optimal, however, because extra hardware is required, and the additional optical signal limits the reach and capacity of the optical transport system. Consequently there is a need for a Raman gain control method that does not employ a probe wavelength.